1. Field of the Invention
The present invention relates to a switching power supply unit having stand-by output.
2. Description of the Prior Art
Conventionally, widely used is a switching power supply unit that is configured to switch a direct current obtained by rectifying and smoothing a commercial alternating current, for example, at high frequency of about 100 kHz and convert the current into a desired voltage by a transformer.
As a system for controlling an output voltage in the switching power supply unit, employed is the system such as a pulse width modulation (PWM) control system that controls a duty ratio of a switching pulse in accordance with changes in the output voltage, a frequency control system or phase control system adopting resonance that controls a frequency or phase of a switching pulse, etc.
In general, in regard to a switching power supply unit included in an electrical apparatus such as a television receiver, or home video system, etc., a main power supply circuit and a stand-by power supply circuit are provided together as shown in FIG. 1, for example, so as to supply a main power supply necessary for the apparatus in an operation state and a stand-by power supply necessary in a stand-by state.
More specifically, a conventional switching power supply unit 200 shown in FIG. 1 comprises a main power supply circuit 240 for main output, which is connected to a commercial alternating current power supply AC through a relay switch SW1, and a stand-by power supply circuit 280 for stand-by output, which is independent of the main power supply circuit 240.
The main power supply circuit 240 is configured to switch the rectified smoothed output provided by a main rectifying and smoothing circuit 210, which rectifies and smooths an alternating current power supply input, by a main converting circuit 220, and rectify and smooth the output of a main converting transformer 225, to which the switched output provided by the main converting circuit 220 is supplied, in a main output circuit 230, thereby obtaining a main output.
The stand-by power supply circuit 280 is configured to switch the rectified smoothed output provided by a sub rectifying and smoothing circuit 250, which rectifies and smooths the alternating current power supply input, by a sub converting circuit 260, and rectify and smooth the output of a sub converting transformer 265, to which the switched output provided by the sub converting circuit 260 is supplied, in a stand-by output circuit 270, thereby obtaining a stand-by output.
The main rectifying and smoothing circuit 210 in the main power supply circuit 240 is constituted by a rectifying circuit 211, a power-factor improvement circuit (PFC) 212, and a smoothing capacitor 213. More specifically, the main power supply circuit 240 supplies a main power supply necessary for the apparatus including the circuit in an operation state, and thus has large output power. For this reason, the power-factor improvement circuit 212 is provided in the main rectifying and smoothing circuit 210 to reduce a high frequency current that exerts a harmful influence on power equipments etc.
On the other hand, the stand-by power supply circuit 280 supplies a stand-by power supply necessary for the apparatus including the circuit in a stand-by state to, for example, a remote control circuit. Therefore, the output power of the circuit is in such a degree that a power-factor improvement circuit is not required, and the sub rectifying and smoothing circuit 250 in the stand-by power supply circuit 280 is constituted by a rectifying circuit 251 and a smoothing capacitor 252.
In the conventional switching power supply unit 200 constructed as described above, in the case that AC input is cut, the lower the AC input voltage is, the smaller the amount of charge stored in the smoothing capacitor 252 of the sub rectifying and smoothing circuit 250 in the stand-by power circuit 280 in which no power-factor improvement circuit is provided becomes in comparison with the amount of charge stored in the smoothing capacitor 213 of the main rectifying and smoothing circuit 210 in the main power supply circuit 240 that comprises the power-factor improvement circuit 212. Therefore, the output from the stand-by power supply circuit 280 stops before the output from the main power supply circuit 240 stops.
Meanwhile, in regard to electronics including a switching power supply unit, in the case that the stand-by power supply is not desired to stop before the main power supply stops, for example, in the case that a system performs an ending process with a stand-by power supply at the time of cutting an AC input, the following measures are taken so that the stand-by power supply does not stop before the main power supply stops.
The capacity of the smoothing capacitor 252 provided in the sub rectifying and smoothing circuit 250 of the stand-by power supply circuit 280 is increased so that the holding time of output in the stand-by power supply circuit 280 becomes equal to or above one in the main power supply circuit 240.
Alternatively, a power-factor improvement circuit is also provided in the sub rectifying and smoothing circuit 250 of the stand-by power supply circuit 280 so that the holding time of output in the stand-by power supply circuit 280 becomes equal to or above one in the main power supply circuit 240.
However, in a normal operation state in that an alternating current supply is supplied, the smoothing capacitor 252 in the sub rectifying and smoothing circuit 250 exceeds the capacity limit, or the sub rectifying and smoothing circuit 250 of the stand-by power supply circuit 280 has an unnecessary power-factor improvement circuit.
Therefore, if such measures described above are taken, they cause harmful effects for miniaturizing, reducing cost, and improving reliability of the switching power supply unit.
To solve the problem described above, an object of the present invention, which has been achieved in view of the situation described above, is to provide a switching power supply unit, which does not have excesses of capacity and a power-factor improvement circuit, which is not fundamentally necessary, and is configured in that the stand-by power supply does not stop before the main power supply does at the time of cutting an AC input.
According to an aspect of the invention, a switching power supply unit is characterized by comprising: a main rectifying and smoothing circuit which rectifies and smooths alternating current power supply input; a main converting circuit connected to the main rectifying and smoothing circuit; a main converting transformer to which the switched output provided by the main converting circuit is supplied; a main output circuit which rectifies and smooths the output from the main converting transformer and obtains main output; a sub rectifying and smoothing circuit which rectifies and smooths the alternating current power supply input; a sub converting circuit connected to the sub rectifying and smoothing circuit; a sub converting transformer to which the switched output provided by the sub converting circuit is supplied; a stand-by output circuit which rectifies and smooths the output from the sub converting transformer and obtains stand-by output; and a back-up circuit which supplies direct current output obtained by the main rectifying and smoothing circuit.